Protective surface covering having heat and moisture resistant properties for caseless ammunition

ABSTRACT

A heat and moisture resistant surface covering for propellant is provided. This surface covering is consumed in the chamber of a gun with the propellant charge being protected by the surface covering. The surface covering is prepared from a polyolefin film base material having a heat resistant and combustible composition bonded to the exterior surface of the base material. The surface covering of this invention has particular utility for use with caseless ammunition.

United States Patent Braak 5] Nov. 28, 1972 PROTECTIVE SURFACE COVERING HAVING HEAT AND MOISTURE RESISTANT PROPERTIES non CASELESS AMMUNITION Edmund C. Blllk, Landing, NJ.

Assignee: Hercules Incorporated, Wilmington,

Del.

Filed: July 7, 1970 Appl. No.: 53,009

Inventor:

US. Cl. ..l02/103, 102/40, 102/D1G. 1 Int. Cl ..F42b l/00, F42b 5/18, F42b 9/16 F1e1dofSear-ch...102/D1G. l, 40, 103, 102, 101, 102/ 104 References Cited UNITED STATES PATENTS 10/1967 Hartz ..102/103 X Holter et a1 102/103 X 3,493,446 2/1970 Braun et a1. 102/103 X 3,557,700 1/1971 Quinlan ..102/10l X 3,557,921 5/1971 Van Langenhoven 102/38 Primary ExaminerRobert F. Stahl Att0meyMichael B. Keehan ABSTRACT 5 Claims, No Drawings PROTECTIVE SURFACE COVFJIING HAVING HEAT AND MOISTURE RESISTANT PROPERTIES FOR CASELFSS AMMUNITION This invention relates to a consumable protective surface covering for propellant. More particularly, this invention relates to a moisture resistant and temperature resistant protective surface covering for propellant which in. the normally finished manufacturing condition is not maintained within a protective case or covering of any kind. Still more particularly this invention relates to a moisture and temperature resistant protective surface covering for protecting a portion or all of the propellant in a round of caseless ammunition.

Propellants employed in some weapon systems are often exposed to atmospheric conditions. Exposure of propellant to varying temperature and humidity conditions can cause deterioration in the ballistic performance of ammunition. Absorption of moisture can also cause propellants to swell. Another major problem which can be encountered with propellant for ammunition in which the propellant is not employed within a protective casing or covering is autoignition of the propellant. This problem can be serious when the propellant is in direct contact with the high temperatures existing in the chamber of a gun in which the propellant is fired, such as during a rapid fire sequence of the gun.

Caseless ammunition comprises a propellant charge, a projectile and a primer. The primer is usually secured within a combustible plug such a a propellant plug which is secured in an axial cavity at the afi-end of the propellant charge. The propellant charge is prepared from molded granules of propellant. The propellant charge is self-supporting, i.e., it is not held within a case of any type. Because of these characteristics, the propellant charge of caseless ammunition is subject to the moisture and autoignition problems described herein.

In accordance with this invention, a heat and moisture resistant protective surface covering for propellant is provided which can be substantially consumed within the chamber of a gun in which the propellant is fired. The surface covering of this invention provides a moisture resistant barrier for the propellant surface covered and has excellent heat resistant properties. The protective covering of this invention comprises a base material of a polyolefin film which covers the propellant. The base material has a heat resistant combustible composition bonded to the surface of the base material. The heat resistant and combustible material comprises by weight from about to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of about 97 percent ethyl acrylate and about 3 percent of a material of the group consisting of combined N-methylol acrylamidezmethacrylamide and combined N-methylol acrylamidezacrylamide in intimate mixture with from about 70 percent to about 90 percent of particulate organic oxidizer of the group consisting of cyclotetramethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX).

Polyolefin films which can be employed in preparing the protective covering of this invention include polyethylene, polypropylene, and compositions or coextrusions of these polyolefins with or without modifying polymers. Mixtures of polyethylene and polypropylene alone or containing modifying polymers such as including other polyolefins, poly( vinyl acetate), poly(ethyl acrylate) and the like can be employed. The polyolefin films employed are from about 0.5 mil to about l.5 mils in thickness. The actual film thickness chosen will depend upon the particular application ofthe protective coating. Thus, for example, when the protective covering is employed for encapsulating propellants employed in weapons using a bag-type propellant charge, the thickness of the film can be up to about 1.5 mils, consistent with consumability considerations for the film and taking into account the conditions existing in the chamber of the gun during a firing. The minimum thickness of the film will be that thickness necessary to provide an adequate moisture barrier. For most applications the polyolefin film thickness will vary from about 0.5 mils to about 0.75 mils.

Polyolefin films employed can be either of the oriented or non-oriented types. This choice again will depend on the application for the protective covering. When employing the film as a total covering for the propellant charge ofcaseless ammunition, adherence of the film to the configuration of the propellant charge is required so that the rounds can be chambered without difficulty. [n this application the polyolefin film is of the oriented type. The methods employed for preparing the protective covering of this invention for this application is fully discussed hereinafter. Non- Oriented films are suitable for use in preparing the protective covering of this invention for bag-type application and the like where adherence of the covering to the precise configuration of the propellant is not required.

Polyolefin film which is employed as the base material in preparing the protective covering of this invention is surface-activated prior to coating of the film with the heat resistant and combustible composition hereinafter described. The purpose of the surface activation treatment is to promote adhesion between the film and the coating composition. Methods known in the art for improving adhesion of materials to polyolefin film, such as, electrostatic or corona discharge treatment, heat and flame treatment ancl chemical treatment with agents such as ozone, gaseous halogen, ultraviolet light, chromic acid and mixtures of sodium dichromate and sulfuric acid can be employed. A preferred method for surface activation of the films is the use of corona discharge. The extent of the surface activation treatment must be such that the polyolefin film does not become blocked. If, for example, the polyolefin film is surface activated to a very high degree with corona discharge, the film if folded upon itself will adhere to itself and can become a solid mass. When such a condition exists the polyolefin film is said to be blocked."

When the polyolefin film employed is of the oriented type and the film is heat shrunk to the configuration of the propellant which is to be protected, it is necessary to prime the surface of the film prior to application of the heat resistant and combustible composition to the film. Printing is necessary since the surface activation of the film is adversely affected by the heat shrinking process. Primers which can be employed are polymeric materials having a polar end group and a non-polar end group. Priming materials are applied to the film in the form of a solution in which the solvent for the priming polymer has a sufficiently low surface tension to effeclarly suitable priming polymer and solvent therefor is available commercially, being sold under the trade 'name Accobond 1066 by American Cyanamid Corporation. This material is a solvent based two-part curable resin. The solvent is ethyl acetate which will not swell or wrinkle the polyolefin film base material. The catalyzed resin is curable in 30 seconds in a forced draft oven at a temperature of 212 F.

The heat resistant and combustible coating composition employed in the protective propellant covering of this invention comprises a binder and an oxidizer. The binder is a crosslinked polyacrylic rubber consisting of about 97 percent ethyl acrylate and about 3 percent of combined N-methylol acrylamide:methacrylarnide by weight. This material is available under the trade name Hycar 2121-X-66 from the B. F. Goodrich Company.

Another suitable binder material is a crosslinked polyacrylic rubber consisting of 97 percent ethyl acrylate and about 3 percent of combined N-methylol acrylamidezacrylamide by weight. The acrylamide or the methacrylamide is usually present in these physical blends in an amount of from about 1 to about 1.5 percent by weight of the binder composition. The binder compositions are used in an amount of from about 10 percent to about 30 percent and preferably from about 10 percent to about 20 percent by weight of the composition.

The heat resistant and combustible coating composition can be employed on the base material in either a cured or uncured state. Curing of the composition is affected by heating the uncured composition at a temperature of about 130 F. for about 16 hours. Higher curing temperatures can be employed and will increase the crosslink density of the composition. Upper temperature limits of curing will depend upon the effect of temperature on the base material and the propellant being surface protected. A crosslinking rate accelerator such as salicylic acid, benzoic acid and phthalic anhydride can be optionally employed. Elimination of the acceleration necessitates a longer cure cycle, but the resulting propellant is essentially the same. From about 0.1 to about 0.5 percent by weight of the accelerator has been found satisfactory.

The oxidizers utilized in the heat resistant and combustible coating composition are cyclotetramethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX) which are organic oxidizers which burn completely, are non-corrosive, and have no noticeable residue. These oxidizers are used in an amount of from about 70% to about 90% and preferably from about 80% to about 90% by weight of the composition. The oxidizers are incorporated in the composition in particulate form and generally have a particle size range of from about 24.4. to about 30p. The heat resistant and combustible composition of this invention can also contain from percent to about 1 percent by weight of wax, such as paraffin wax, based on the weight of the composition.

The heat resistant and combustible compositions employed to coat the polyolefin film are prepared using conventional smokeless powder equipment and using the method of preparation as set forth in US. Pat. No. 3,152,027 wherein volatile solvent is utilized for desensitizing the doughy mixture of binder and particulate oxidizer. Illustrative compositions employed in preparing the moisture and heat resistant protective covering of this invention are set forth in Table 1 below:

TABLE I Composition 1 2 3 4 Weight percent:

HMX (Class B) 84.00 72.00 84.00 RDX (Type E) 72.50 Hycar 2l2l-X-66 l5.84 15.84 27.20 15.84 Salicylic Acid 0.16 0. lo 0.30 Oxamide 12.00 Phthalic Anhydride 0. l6

Totals l00.0 103.0 100.0 100.0

Sieve Size (mesh) Percent through Additional data on these combustible compositions is set forth in US. Pat. No. 3,386,868 to J. R. Gimler et al., reference to which is hereby made.

The heat and moisture resistant protective surface covering of this invention is prepared by applying a solution-suspension of the heat resistant and combustible composition to a base material of polyolefin film. This solution-suspension, referred to hereinafter as a suspension, can contain up to about 50 percent and preferably from about 20% to about 30% by weight of the heat resistant and combustible composition based on the weight of the suspension. Diluents or suspending agents for the heat resistant and combustible compositions include acetone, methyl ethyl ketone, methyl isobutyl ketone, ethyl ether, ethyl acetate, benzene, trichloroethylene, toluene, ethanol, the xylenes and mixtures thereof. The particular diluent or mixture of diluents employed must be evaluated in terms of the effect of the diluent on the tendency of the coating composition to crack or the base material to swell during diluent removal. [I is generally desirable to employ a diluent having a high evaporation rate, making removal of the diluent from the surface of the coated base materials as easy as possible. A particularly suitable diluent for use with a polypropylene film as the base material is a mixture comprised of about percent by weight of methyl ethyl ketone and about 5% by weight of toluene.

A method employed to provide a propellant with the moisture and heat resistant surface covering of this invention is set forth broadly as follows. A suspension of the heat resistant and combustible composition is applied to the exterior of the surface activated base material such as by knife coating or spraying so that a coating of substantially uniform thickness adheres to the film surface. Diluent is removed from the surface of the coated base material by any suitable means such as by drying in an oven or in a stream of warm dry air. The coated base material is heat sealed to form a bag. The propellant is charged into the bag and the bag is heat sealed again encapsulating the propellant.

A suitable method for applying the protective surface covering of this invention to the entire propellant charge of a round of caseless ammunition comprises as a first step encapsulating the propellant charge or the entire caseless round, depending on the round configuration, with a base material which is an oriented polyolefin film. .The oriented film is then heat-shrunk to the configuration of the propellant charge or round. A primer is applied to the exterior surface of the heatshrunk film. A suspension of the heat resistant and combustible composition is then applied to the exterior surface of the heat shrunk base material. This coating composition is applied to the exterior surface of the primed base material preferably by immersing the en- I capsulated propellant into a bath containing a suspension of the coating composition and slowly withdrawing the encapsulated propellant from the bath. The weight ratio of the coating composition to the bat: material is generally from about 5/1 to about 8/ l and the thickness of the applied coating composition is preferably from about 2.5 mils to about 4.5 mils. Within this thickness range the base material will be substantially consumed in the chamber of most guns during firing. By substantially consumed" is meant that less than about 0.5 percent by weight residue based on the weight of the protective surface covering remains after a firing. Illustrative rounds of caseless ammunition which can be protectively covered employing the methods heretofore described are completely described in US. Pat. No. 3,482,516 and US. Pat. No. 3,345,945 reference to which is hereby made.

The following examples more fully illustrate the protective covering of this invention. All parts and percentages are by weight unless otherwise specified.

Example 1 illustrates a simple process for preparing the protective surface covering of this invention for use in covering propellant employed as a mortar charge.

EXAMPLE I i A suspension of heat resistant and combustible coating composition is prepared by mixing 40 parts of a composition comprising 84 percent cyclotetramethylenetetranitrarnine, 15.8% Hycar 2l2l-X-66 and 0.16 percent salicylic acid in 60 parts of a solution of 95:5 parts methyl ethyl ketone: toluene. The coating composition is then knife coated to a sheet of non-oriented polyethylene film of 0.75 mil thickness which has been activated on one side by corona discharge. The coated film is heat sealed into ba about 1 inch wide by 4 inches long. About parts of M-9 propellant for an 81 mm mortar is placed in the bag, and the bag is heat sealed to encapsulate the charge.

The following example illustrates the process of encapsulating a round of caseless ammunition with the protective covering of this invention.

EXAMPLE 2 A polypropylene bag 2% inches wide, 12 inches long and open at one end is formed in one operation from 0.5 mil polypropylene film by fusion sealing with a Weldotron Magna-Lock L-Sealer. The film is biaxially oriented and subjected to corona discharge on one side of the film. A round of caseless ammunition having a length of 5.96 inches and an outside diameter of 1.53

inches is inserted in the bag. The bag is fusion sealed to a final length of 8.0 to 8.5 inches. A small pin hole is made in each end of the sealed bag to permit air to escape during the'shrinking process. The bagged round is exposed to high velocity air at 370 to 380 F. for from 6 to 8 seconds in a Weldotron shrink tunnel. This procedure shrinks the film tightly and wrinkle free about the exterior surface of the round without rupturing the seams. The comers of the bag shrink to the end configuration of the round. These corners are flattened through alternate heating and cooling under pressure. 'lhe air-escape pin holes at each'end of the bag are sealed with one-eighth inch squares of 1 mil polypropylene tape. The film surface is primed by dipping the encapsulated round into a 5 weight percent solution of Accobond 1066. The primed round is dried in air at ambient temperature for about 10 minutes. A two-pronged brass spring clamp is attached to the aftend of the round. The round is immersed in about a 40 percent suspension of the heat resistant and combustible composition as described in Example 1. The encapsulated round is withdrawn from the suspension using a Fisher-Payne dip coater, at a rate of 2 inches/min. The resulting protective covering is odor free and is dry to the touch after 10 minutes of drying at ambient temperature.

The following examples illustrate the moisture resistance of the protective covering of this invention evaluated on caseless ammunition. The caseless rounds are comprised of a propellant charge, a projectile and a combustible plug containing a primer for said caseless rounds. The primer plug is secured in an axial cavity in the aft-end of the propellant charge. In these examples propellant charges for caseless rounds are prepared from molded granules of double-base type smokeless powder.

EXAMPLES 3-24 In Examples 3-12 rounds of caseless ammunition are encapsulated in the protective surface covering of this invention as described in Example 2. Rounds of Examples 8-12 are subjected to temperature and humidity cycling.

ln Examples 13-16 only a portion of the rounds of caseles ammunition are protected with the surface covering of this invention. In these rounds the primer plug for the rounds of caseless ammunition containing a priming composition and optionally containing a booster charge is shrink wrapped with polyolefin base material. The primer plug is inserted into and secured in a cavity in the aft-end of the propellant charge of each caseless round. The assembled round is dip coated with the heat resistant and combustible composition prepared in Example 1 to which composition has been added 1.2% by weight of parafi'rn wax. These rounds are subjected to temperature and humidity cycling and fired.

Rounds 17-24 are control rounds. Rounds 17-20 are coated with the coating composition only (no base material employed). Rounds2l-24 have no protective covering whatsoever. These rounds are subjected to ples 3-12 or it can be used effectively as a surface covering for a portion of a propellant charge such as the aft-end of the primer plug as illustrated in Examples 13-16.

temperature h m d t l R ult f fi all the 5 What l claim and desire to protect by Letters Patent rounds is set forth in Table 11. Is:

'lAliLE ll 24-hour 'lotnl lmnpl-rn- (Jul. nctlon [teslrluo (Jun-go Innmu! Wright mnzzlu Ulnnnlmr Limo welglit ltrzunplv Protective weight hnmldlly gnln velocity 2 pressure (mllll- (millimund covering (grams) ryvlvs l (grams) (loot/sue.) hsz/lnfl) surnmls) grnlns) 1H. -1 l) (l --l78 ill, 30(1 27 Ill 18. 7 I) (l 173 Ill), 0011 .30 H 22. U 1] -12!) "2, 501) :0 l2 .fll. L! (l (1 -11!) 67, 300 .52 lli 2n 0 (1 l88 Ml, 100 35 13 IN. -l H) l. .211 3ll .800

T. 5 ll] 1. l3 -3'.-1 48, D00 :0. n 1. 05 -243 53, 200 11!. 10 1.08 2ti1 54, 200 1h. 10 1.08 -21i3 51, 100 .53. 10 1. iii! -33: 45, 201) ll) 1. 9G -452 41,401) 10 l. 80 306 4'1, 800 ll] 2. 43 -332 4b, U00 ll) 2. ()8 473 38, 000 10 .2. 33 486 3!), 400 ll) .5. [)2 -2J0 45, ($01] 11) 2. b3 -3.|8 3!), 700 11) (i. 41) 2270 20, 200 10 4. 70 220(] .20, 700 10 IS. 62 10 4. 70 1 Ten 24-hour cycles between 70 and 105 F. at J5 to 100% RH. according to 'IECI 700-700, Interim Pamphlet 2 Velocity reported as difference from design velocity.

3 Total cumulative residue.

4 Primer fired, rounds did not fire.

5 Only aft-end 0f plug containing priming 6 Rounds coated with heat resistant composition only.

.. 7 l-Zssm ds The moisture gain of the rounds of caseless ammunition which are fully protected with the surface covering of this invention (Examples 8-12) is substantially reduced compared to the moisture gain of uncoated rounds (Examples 21 24).

Rounds 13-16 l6having the surface covering over the aft-surface of the primer plug all fire. Action times for these rounds is satisfactory. Rounds 1? coated with the heat resistant and combustible coating only have unacceptable action times.

Ballistic performance of rounds 21-24 which are not protected with the surface covering of this invention is unsatisfactory. Rounds 1? and 18 fire but action,time is not satisfactory. Rounds 19 and 20 fail to fire at all.

EXAMPLE 27 A molded propellant charge is prepared from double base propellant. This charge is covered with the protective surface covering of this invention. Autoignition ofthe surface protected charge is measured by pressing the charge against a steel surface having constant surface temperature of 540 F. The propellant charge ignited after 2.5 seconds of contact with the steel surface. As a control, another molded charge of the same double base propellant composition is subjected to the same autoignition test. This propellant charge has no surface covering. This propellant charge ignites after 1.4 seconds in contact with the steel surface at a temperature of 540 F.

From the foregoing examples it is evident that the moisture and heat resistant protective surface covering of this invention can be used to protect all, or any portion of a piece of propellant employed as a propellant charge. Thus, the moisture and heat resistant protective covering of this invention can be employed to totally encapsulate propellant as illustrated in Exam- Army Test and Evaluation Connnand Material Test Procedures).

composition protected with surface covering.

1. A heat and moisture resistant protective surface covering for propellant capable of being substantially consumed within the chamber of a gun with the propellant, said protective surface covering comprising a base material of a polyolefin film having an activated exterior surface and a heat resistant and combustible composition bonded to the exterior surface of said base material, said heat resistant and combustible coating material comprising by weight from about 10 percent to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of about 97 percent ethyl acrylate and about 3 percent of a material of the group consisting of combined N-methylol acrylamidezacrylamide and combined N-methylol acrylamidezmethacrylamide in intimate mixture with from about percent to about percent of particulate organic oxidizer of the group consisting of cyclotetramethylenetetranitramine and cyclotrimethylenetrinitramine.

2. The heat and moisture resistant protective surface covering of claim 1 wherein the base material is polyethylene film or polypropylene film.

3. The heat and moisture resistant protective surface covering of claim 2 wherein the base material has a thickness of from about 0.5 mils to about 1.5 mils.

4. The heat and moisture resistant protective surface covering of claim 3 wherein the heat resistant and combustible coating composition is applied to the base material to a thickness of from about 2.5 to about 4.5 mils, the weight ratio of said coating to the base material being from about 5/1 to about 8/1 5. A heat and moisture resistant protective surface covering for propellant capable of being substantially consumed within the chamber of a gun with the propellant, said protective covering comprising a base material of polypropylene film having a thickness of from about 0.5 to 0.75 mils and having an activated exterior surface, and a heat resistant and combustible composition bonded to the activated exterior surface of the polypropylene film, said heat resistant and combustible composition comprising by weight from about 10 percent to about 30 percent of a croealinked polyacrylic I 

2. The heat and moisture resistant protective surface covering of claim 1 wherein the base material is polyethylene film or polypropylene film.
 3. The heat and moisture resistant protective surface covering of claim 2 wherein the base material has a thickness of from about 0.5 mils to about 1.5 mils.
 4. The heat and moisture resistant protective surface covering of claim 3 wherein the heat resistant and combustible coating composition is applied to the base material to a thickness of from about 2.5 to about 4.5 mils, the weight ratio of said coating to the base material being from about 5/1 to about 8/1
 5. A heat and moisture resistant protective surface covering for propellant capable of being substantially consumed within the chamber of a gun with the propellant, said protective covering comprising a base material of polypropylene film having a thickness of from about 0.5 to 0.75 mils and having an activated exterior surface, and a heat resistant and combustible composition bonded to the activated exterior surface of the polypropylene film, said heat resistant and combustible composition comprising by weight from about 10 percent to about 30 percent of a crosslinked polyacrylic rubber binder consisting essentially of about 97 percent ethyl acrylate and about 3 percent of combined N-methylol acrylamide:methacrylamide in intimate admixture with from about 70 percent to about 90 percent of particulate cyclotetramethylenetetranitramine (HMX). 